The development of surgical techniques have made great progress over the years. For instance, for patients requiring brain surgery, non-invasive surgery is now available which is afflicted with very little trauma to the patient.
One system for non-invasive surgery is the Leksell Gamma Knife® Perfexion system, which provides such surgery by means of gamma radiation. The radiation is emitted from a large number of fixed radioactive sources and is focused by means of collimators, i.e. passages or channels for obtaining a beam of limited cross section, towards a defined target or treatment volume. The collimator comprises a number of sectors where each sector may be set in a number of different states or collimator passage diameters. Typically, a collimator comprises eight sectors and each sector can be set in four different radiation states (beam-off, 4 mm, 8 mm, and 16 mm). It is possible to individually adjust the sectors, i.e. select a specific state for a specific sector, to change a spatial dose distribution surrounding the focus point, which is the point of convergence focused by the collimator.
Treatment planning optimization for radiation therapy, including for example gamma knife radiosurgery, aims providing a sufficient dose to the target volume within the patient (e.g. in treatment of tumours) at the same time as the dose delivered to adjacent normal tissues is minimized in order to spare healthy tissue and organs. In treatment planning optimization, the delivered radiation dose is limited by two competing factors where the first one is delivering a maximum dose to the target volume and the second one is delivering the minimum dose to the surrounding normal tissues.
A further important parameter which has to be taken into account is the total treatment time. The treatment planning optimization will result in a number of isocenters and for each isocenter, the position of the shot within the target volume to be treated as well as the beam-on time for each sector and for each state of each isocenter. Typically, the beam-on time for the eight different sectors and the four states (beam off, 4 mm, 8 mm, and 16 mm) are determined for each isocenter. A treatment including irradiation with each sector in each state of each isocenter would, in most cases, lead to very long irradiation times and thereby very long treatment times, which is uncomfortable for the patient, leads to a low patient flow and may, in addition, have adverse biological effects due to the low average doserate of the dose delivery.
Accordingly, there is need for more efficient methods and procedures for increasing the quality of the optimized treatment plan by shortening the treatment time.